Method of fabricating a pressure sensor

ABSTRACT

A pressure sensor including a pressure capsule such as a dual diaphragm quartz capacitance pressure capsule and structure for supporting and sealing same within the pressure environment to be measured. The pressure capsule having two parallel quartz plates spaced by a dielectric ring forming a vacuum chamber therein. The structure providing a force balanced support for compressively loading the pressure capsule through the dielectric ring. The structure further having transfer port therein to communicate the pressure environment to both plates. 
     The method of making the quartz capacitive pressure capsule includes the steps of surface preparation, preglazing of the component parts and vacuum sealing by utilizing a controlled pressure-temperature profile. 
     A registration fixture is described which contains a plurality of flat plates, each plate having a pressure capsule receiving cavity sized to closely receive one of the quartz plates and further provides a uniform temperature radiation pattern for sealing of the pressure capsule during the controlled pressure-temperature profile.

This is a continuation of application Ser. No. 30,588 filed Apr. 16,1979, now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method of fabricating a quartz capacitancepressure capsule. More particularly, the invention relates to a methodof surface preparation, electrode curing and vacuum sealing of theparallel plates of the pressure capsule.

The incorporation of pressure sensors into the electronic controlsystems of automotive vehicle poses severe operational requirements forthe pressure sensor. These requirements are further heightened when thepressure sensor is used to measure the manifold pressure (MP) or theabsolute manifold pressure (MAP) of the engine.

The pressure sensor must operate in a mobile and severely and hostileenvironment which may include any of the following characteristics:varied temperature extremes, excessive shock and vibration and highlevels of radiation causing electromagnetic interferences and corrosivegases and liquids. The pressure proximate the intake manifold is rapidlychangeable and susceptable to large variations in magnitude (1-4atmospheres) which may be caused as the result of explosive backfire ormay occur if the vehicle is so equipped during the boost phase of theoperation of a supercharger or turbocharger; thus requiring a pressuresensor having a large dynamic range and high sensitivity. Irrespectiveof the above, if the present invention is utilized in the automobile,the requirements of the industry dictate that it must be (1)inexpensive; (2) repeatable; and (3) capable of being mass producedwhich implicityly requires the use of novel and inexpensive fabricationtechniques as opposed to the ion-milling, vacuum deposition methods suchas sputter-etching techniques and brazing fabrication steps mentioned byPolye in U.S. Pat. No. 3,858,097, and by Dias, et al in U.S. Pat. No.4,064,550.

The present invention is a method of fabricating a dual diaphragm quartzcapacitance pressure capsule. The pressure capsule comprises a pair offlat flexible fused quartz plates which are separated by a ring ofdielectric material (such as a glass frit) defining an interior chamberwhich is maintained at a determinable pressure (vacuum) reference level.The pressure capsule contains a plurality of electrodes located withinthe interior chamber forming the conducting plates of a referencecapacitor C_(r) and a pressure sensing capacitor C_(p). In particular,one plate, the upper plate contains a ground electrode while the otherplate, i.e., the lower plate contains both the C_(p) and C_(r)electrodes. The lower plate may also contain an electrical shield on asurface opposite the internal chamber. In addition, the pressure capsulecontains a plurality of electrical contacts, one associated with each ofthe above electrodes. These contacts are located outside of the internalchamber near the edges of each of the flat plates. The method comprisesthe steps of (a) preparing the surface of each quartz plate to adeterminable flatness; (b) printing the electrodes and electricalcontacts thereon using a metal organic ink; (c) curing the electrodes byheating and controlling the temperature profile to remove the organicbinders within the ink and to cause the remaining metals to bind oradhere to the quartz plates; (d) printing a slurry glass frit, i.e., thedielectric ring thereon; (e) preglazing the frit to remove non-glassmaterials in the frit to minimize the formation of a bubble structuredeveloped in later steps; (f) assembling the quartz plates within aregistration fixture to form an unsealed pressure capsule or pluralityof capsules; and (g) vacuum sealing the unsealed capsule comprising thesteps of varying the local temperature and pressure in accordance with adescribed temperature-pressure profile shown in FIG. 12.

In response to pressure input thereto, both plates act as cantileveredplates, and deflect and bend over the raised dielectric ring varying thecapacitance between the plates.

The housing includes means for circumferentially sealing andcompressively supporting both plates wherein the compressive forces onthe plates are opposingly directed through the dielectric ring. Thehousing further includes means, such as a port, for communicating thepressure to be sensed to the upper plate proximate the general locationof the interior chamber and transfer port means located therein tofurther communicate the pressure to the second plate.

An advantage of the present invention is that capsule to capsuleuniformity is enhanced by minimizing developed bubble structures. Inaddition, the depositing of electrodes and dielectric material by silkscreening permits the achievement of a low cost fabrication process. Afurther advantage of the process is that pressure capsules havingdifferent spacing i.e., capacitance can be achieved. This can be seenfrom the following: During the vacuum sealing of the capsule, thedielectric is caused to return to a molten, viscous state and byincreasing the local pressure, a clamping force is applied to bothplates. It can be appreciated that the spacing therebetween will varywith the maximum applied pressure (force) and its time of application.

It is an object of the present invention to fabricate quartz capacitorshaving a uniform bubble structure in its electrodes and dielectriccomponents.

It is an object of the present invention to seal a pressure capsule in acontrolled temperature pressure (vacuum) environment.

It is an object of the present invention to provide a pressure capsulehaving an internal pressure (vacuum) reference and to eliminate the needfor vacuum tip-offs.

It is a further object of the present invention to eliminate the needfor heavy hold down weights and large thermal masses used during thesealing process.

It is an additional object of the present invention to provide a methodof fabrication adaptable to the mass production of quartz pressurecapsules.

These and other objects, features and advantages of the invention willbe clear from the detailed description of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an exploded view of the preferred embodiment of a pressuresensor.

FIG. 2 is a sectional view of the pressure sensor.

FIG. 3 is a sectional view through Section 3--3 of FIG. 2 with portionsof the pressure sensor removed.

FIG. 4 is a sectional view taken through Section 4--4 of FIG. 2 withhousing 22 removed.

FIG. 5 is a partial sectional view through Section 5--5 of FIG. 4.

FIG. 6 is a sectional view of the pressure capsule.

FIG. 7 is a bottom view of the upper disc of the pressure capsule takenthrough Section 7--7 of FIG. 6.

FIG. 8 is a top view of the lower disc of the pressure capsule takenthrough Section 8--8 of FIG. 6.

FIG. 9 is a sectional view of a registration fixture.

FIG. 10 is a sectional view through Section 10--10 of FIG. 9.

FIG. 11 is a sectional view through Section 11--11 of FIG. 9.

FIG. 12 illustrates the electrode curing process.

FIG. 13 illustrates an alternate electrode curing process.

FIG. 14 illustrates the temperature-pressure sealings process.

FIG. 15 illustrates an alternate embodiment of the pressure capsule.

FIG. 16 illustrates a sectional view taken through Section 16--16 ofFIG. 15.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is made to FIG. 1, which is an exploded view of the preferredembodiment of the capacitive pressure sensor 20. In particular, FIG. 1illustrates the interrelationship of the primary components of thepresent invention. There is shown a cup-like housing 22, a base 36 andcover 38. The housing 22 and base 36 combine to protect, support andenclose a pressure capsule 100 which is suspended therebetween by a pairof o-rings 62 and 64. The base 36 is designed to seat against acircumferential shoulder 34 within the housing 22. The signalconditioning electronics 150 are suspended between the base 36 and cover38 by three pedestals. Two of these pedestals 88a and b are integralparts of the base 36. The third pedestal is an electrically conductivepost 90 which projects from the housing 22 and extends through acentering groove 92 in the base 36 providing the third leg of the threelegged support for the signal conditioning electronics 150. Signalcommunication with the electronics 150 is through a plurality of pins 66in the base 36. As will be discussed later, post 90, in addition tosupporting the electronics, provides an improved electrical groundconnection linking the housing 22, base 36 and electronics 150. In thepreferred embodiment, the housing 22 is fabricated of an electricallyconductive material such as aluminum, or carbonized plastic whichcooperates with the components of the pressure capsule 100 to isolatepressure measurements from stray electric fields. The base 36 maysimilarly be fabricated of metal or alternatively of a non-conductivematerial such as a thermo-set plastic or carbonized plastic having therequired number of electrical pins 66 extending therethrough. These andother features of the present invention will be discussed in more detailin the accompanying figures.

FIG. 2 is a sectional view of the assembled pressure sensor 20, furtherillustrating the relationship between the housing 22, base 36, pressurecapsule 100 and its associated signal conditioning electronics 150. Thehousing 22 includes a bottom 24 and a walled portion 26 protrudingtherefrom defining a cup-like cavity 28. The housing 22 has twocircumferential shoulders 32, 34 surrounding the cavity 28, located onthe interior portion of the cylindrical wall 26 and spaced from thebottom 24 of the housing 22. The shoulder 34 provides a means forcircumferentially supporting the base 36. The base 36 is a disc-likemember comporting to the substantially circular dimensions of theinterior of wall 26 and to the dimensions of the circumferentialshoulder 34. The exterior dimensions of the base 36 may be chosen toprovide for a press fit engagement between the interior of the wall 26and the base 36 or may be sized to be staked in place at the base36-shoulder 32 interface (staking not shown). When the base 36 ismounted within the housing 22, it is spaced from the bottom 24 forming apressure receiving cavity 44 therebetween. The pressure receiving cavity44 is the innermost portion of the cup-like cavity 28. The pressurereceiving cavity 44 is vented to an external working pressure through aninput passage 46 located within the bottom 24 of the housing. Asillustrated, the input passage 46 is located within a pressure receivedport 48 which protrudes from the bottom 24 of the housing 22.

The housing 22 and base 36 cooperate to support the pressure capsule 100within the pressure receiving cavity 44. The housing 22 is fabricatedwith a circular groove 50 formed in a surface 52 of the bottom 24adjacent to the pressure receiving cavity 44. In the embodiment shown inFIG. 2, the groove 50 is coaxial to an axis 54 which runs through thecenter of the input passage 46. In addition, the base 36 is fabricatedwith a similar groove 56 which is located on a surface 58 of the base 36adjacent to the pressure receiving cavity 44. In the assembled statewith the base 36 seated upon the shoulder 34, the second groove 56 willbe located coaxial to axis 54 and immediately below but spaced apartfrom groove 50. The significance of this spacing will be discussedlater. It is a requirement of the invention that grooves 50 and 56 be inregistration with one another, thus necessitating a means of aligningboth grooves. This is done in the preferred embodiment in a straightforward manner and is accomplished by controlling the tolerances betweenthe base 36 and the interior of the wall 26, i.e. shoulders 32 and 34.The groove 56 is formed in the base 36 concentric to its edge 60 thereinpermitting the required accuracies upon assembly. The coacting set ofgrooves 50 and 56 are sized to accept identical pairs of o-rings 62 and64 for supporting the pressure capsule 100. It can be seen that byrequiring the alignment of the grooves 50 and 56 the clamping orcompressive force exerted on the pressure capsule 100 will be directedsubstantially through the centers of the grooves 50 and 56. The degreeof compressive force exerted on the pressure capsule 100 is determinedby the spacing between the parallel surfaces 52 and 58. The o-rings 62and 64 in cooperation with the base 36 and housing 22 provide a meansfor resiliently supporting the pressure capsule 100, and means for forcebalancing the pressure capsule 100 wherein equal and oppositecompressive forces are imparted to opposing surfaces of the pressurecapsule 100. The o-rings (62, 64) also provide a pressure tight seal forthe pressure receiving cavity 44 which is exposed to the workingpressure environment, therein isolating it from the remaining portionsof the cup-like cavity 28. The isolation of the pressure receivingcavity 44 has further significance in the following context. As will bedescribed later, the pressure capsule 100 is a capacitive pressurecapsule comprising two parallel quartz plates (102, 104) having aplurality of electrodes (110, 112, 114) imprinted thereon. Electricalcommunications between the signal conditioning electronics 150 and thepressure capsule 100 are through a plurality of electrical contacts(128, 146) which are located about the periphery of the pressure capsule100. Thus, as shown, the o-rings (62, 64) isolate these electricalcontacts from the working pressure environment. This is quitesignificant as can be seen by the following: One use of the presentinvention is to measure the absolute pressure in the intake manifold ofan internal combustion engine. In this context corrosive gases will beinput into the pressure receiving port 48. In addition, if the enginehas a turbo or supercharger, there may be intervals during the operationof the engine wherein raw fuel will be input into the pressure receivingport 48, therein further illustrating the need to isolate the electricalcontacts from the pressure receiving cavity 44. Electrical communicationbetween the electrical contacts (128, 146) of the pressure capsule 100and its associated pressure sensor electronics 150 is through aplurality of electrically conductive pins 66 which are imbeded in andextend through the base 36. Additional sealing between the base 36 andhousing 22 and protection for the electronics 150 can be achieved byfilling that portion of the cavity 28 which houses the electronics 150(between the base 36 and cover 38) with a sealing material such asHumiseal manufactured by Columbia-Chase, Woodside, N.Y.

As previously discussed, the present invention provides the workingpressure environment to both surfaces of the pressure capsule 100. Thisis accomplished by providing the base 36 and bottom 24 with a pluralityof intersecting passages 78a, b, c which together comprise a transferport 80 for transferring the working environment to both pressurereceiving surfaces of the pressure capsule 100. Inasmuch as the pressurewithin the transfer port 80 will be that of the working pressureenvironment it is necessary to provide a pressure tight seal about thatportion of the transfer port 80 at the juncture of the base 36 andshoulder 34. This is accomplished by providing the base 36 with a groove82, coaxial to a center line 84 through the passage 78b, which is sizedto accept an o-ring 86. In this manner upon assembly, the base 36 willcompress the o-ring 86 against the shoulder 34 of the housing 22 thereinproviding a pressure tight seal of the transfer port 80. Similarly,passages 78a and 78c must also be sealed. The required pressure tightseal can be accomplished by welding, sealing or epoxying at locations 68at the ends of the respective passages 78a and 78c.

Reference is now made to FIG. 3 which is a sectional view taken throughsection 3 of FIG. 2 with the base 36, pressure capsule 100 and o-ring 62removed. The post 90 has also been omitted to show the post receivinghole 70 more clearly. FIG. 3 further illustrates the location of thetransfer port 80 formed by passage 78b.

Reference is made to FIG. 4 which is a sectional view taken throughSection 4--4 of FIG. 2 with the housing 22 removed and furtherillustrates the mounting relationship between the pressure capsule 100and the base 36. A detailed description of the pressure capsule 100 canbe found in the discussion of FIGS. 6 through 8. It is sufficient fordiscussion of FIG. 4 to visualize the pressure capsule 100 as asubstantially circular structure which is coaxially mounted relative tothe center of base 36 and axis 54 of the housing 22. FIG. 4 alsoillustrates the relationship between the electrical contacts (128, 140,146) of the pressure capsule 100 and the plurality of electrical pins 66extending through the base 36. The contacts 128, 140, and 146 areconnected to pins 66a, b, and c through a plurality of electrical leads138a, b, and c. The top view of the base permits easy recognition of aportion of the transfer port 80, in particular, passage 78b, with itscoacting pressure tight seal, i.e. o-ring 86, located within groove 82.In addition, the alignment slot 92, the relationship of groove 56 to itscoacting o-ring 64 and to the geometries of the pressure capsule 100 arereadily discernable.

FIG. 5 is a side view of the base 36 with the electronics 150 removedand also contains a partial sectional view illustrating the details ofthe transfer port 80. FIG. 5 more clearly illustrates the relationshipof the pedestals 88a and b and the relationship of pins 66a through 66cto the other components of the base.

FIGS. 6, 7, and 8 illustrate the details of the pressure capsule 100.The pressure capsule 100 comprises a dual diaphragm having two coaxiallyoriented oblong non-conductive flexible plates or discs which arepreferably fabricated from fused quartz. As will be described later,each plate or disc of the assembled capsule 100 is rotated relative toone another about their common centers 108. The pressure capsule 100consists of an upper plate or disc 102 and a lower plate or disc 104.Each disc is maintained in spaced relationship one to the other by adielectric material such as a frit glass 106. The upper and lower discs102 and 104 have the same physical shape and differ by the electrodes110, 112, 114 and a ground shield 116 imprinted thereon. In thepreferred embodiment, as illustrated in FIGS. 6-8 each quartz disc is asubstantially circular member having flattened sides giving it asubstantially oblong appearance. The flattened sides 132 and 150 may beviewed as cut-outs and provide a reference for alignment and forregistration permitting the accurate placement of the electrodes anddielectric material thereon. The flattened sides 132, 150 or cut-outsalso provides easy access to a plurality of electrical contacts 128, 140and 146. In addition, the sides 132 and 150 or cut-outs in co-operationwith a registration fixture 175, discussed in FIGS. 9, 10 and 11,provide a means for aligning the plates or discs 102 and 104 at adeterminable orientation relative to each other prior to and duringfabrication. Other plate shapes and cut-outs may be substituted incomformity with the teachings herein. The upper quartz disc 102 has anupper surface 120 and a parallel lower surface 122. In a similar manner,the lower quartz disc 104 has an upper surface 124 and a parallel lowersurface 126. A substantially circular ground electrode 122 having twooppositely extending electrical contacts 128a and b is disposed on thelower surface 102. These electrical contacts extend across the largerdimension of disc 102 to its edge 130, as shown in FIG. 7.

Reference is made to FIG. 8 which illustrates the details of the lowerquartz disc 104. The lower quartz disc 104 has a circular electrode 114disposed on its upper surface which is electrically connected to itsassociated electrical contact 140. The electrical contact 140 is locatedon the circular or peripheral edge 142 of the lower quartz disc 104. Inaddition, the upper surface 124 contains a substantially circular ringor c-shaped electrode 112 which partially surrounds the smallerelectrode 114. The c-shaped electrode 112 is in electrical communicationwith an associated contact 146 located at the periphery of the lowerdisc 104 and is oppositely situated relative to contact 140. Furtherinspection of FIG. 8 reveals the c-shaped outer electrode 112 isconcentricly disposed relative to the inner electrode 114 and toelectrode 110. The electrode combination 110-114 shall be designated asa pressure sensing capacitor C_(p) while the electrode combination110-112 shall be designated as the reference capacitor C_(r). Thediameter of the ground electrode 110 should be chosen equal to orgreater than the diameter of the outer electrode 112. In the preferredembodiment, the ground electrode 110 has a diameter which is ten percentlarger than the diameter of the outer electrode 112 and the areas ofelectrodes 112 and 114 are made equal yielding capacitances C_(p) andC_(r) which are substantially equal. The purpose of requiring the groundelectrode 110 to be equal to or slightly greater than the diameter ofthe outer electrode 112 is to prevent the introduction of straycapacitance into the electrical measurements providing a degree ofelectromagnetic isolation for the pressure capsule 100. It can be seenthat the capacitive capsule 100 is effectively shielded fromelectromagnetic radiation by the combined shielding effects of theground electrode 110 and ground shield 116. In addition, the groundelectrode 110 and ground shield 116 may be connected together by using awire such as ground link 118 therein insuring that the ground electrode110 and ground shield 116 remain at the same electric voltage potential.

FIG. 8 further illustrates the relationship between the dielectric fritglass 106 and the electrodes 110, 112 and 114. In particular, the fritglass 106 which spaces the two discs apart from one another is acircular ring and has a diameter greater than either of the diameters ofelectrodes 110, 112. In addition, the dielectric frit glass 106 forms apressure tight seal for the interior chamber 148 therebetween. If thepressure capsule is to function as an absolute pressure sensingapparatus the interior chamber 148 must be evacuated. The process ofevacuating the interior chamber is discussed in detail later. The degreeof vacuum within the interior chamber 148 will depend upon the low rangeof the desired pressure sensitivity. Under certain circumstances, itmight even be desirable to back fill the interior chamber 148 to apredetermined pressure or with an inert gas to achieve a degree oftemperature compensation. Reference is again made to FIG. 6 whichillustrates by use of phantom lines the relationship between o-rings 62and 64 and the circular ring of frit glass 106. To achieve a forcedbalanced situation, it is required that the compressive forces exertedon the pressure capsule 100 by o-rings 62 and 64 be circumferentiallyapplied directly above and below the frit glass 106.

Further inspection of FIGS. 6-8 reveal a unique technique, not limitedto a capacitive capsule, of arranging the plurality of electricalcontacts needed to communicate with the pressure capsule 100. Inasmuchas each quartz plate or disc (102, 104) has an oblong shape it isdesirable to place the electrodes on the appropriate quartz disc at thedimensions of maximum distance from the center of each plate or disc. Byrotating quartz plate 104, a determinable amount such a determinableamount such as 90° relative to quartz disc 102 it is apparent that theelectrodes, which are located at the maximum distances from the centerof each of the respective discs, will extend beyond the smallerdimensions of the substantially oblong discs. As previously mentioned,the preferred embodiment uses a substantially circular quartz dischaving flattened sides, i.e. sides 132, 150, permitting the electrodesto extend beyond these flat areas for easy access.

The upper surface 120 of the upper quartz disc 102 and the ground shield116 or the lower surface 126 of the lower disc 104 can be thought of astwo pressure receiving surfaces and the areas of these pressurereceiving surfaces interior to the diameter of the glass frit 106 mayfurther be described as pressure responsive regions. The capacitiveplates or discs 102, 104 will tend to deflect upon the application ofthe normal component of the pressure force being sensed at pointsinterior to the support provided by the glass frit 106. A pressurecapsule 100 having two pressure responsive regions yields a sensorhaving increased sensitivity when compared to a sensor employing asingle pressure sensitive surface, that is, the use of two pressureresponsive surfaces permits the use of a smaller pressure capsule 100 toachieve the same change capacitance output in comparison to a pressuresensor employing a single pressure responsive surface.

The deflections of the discs 102, 104 will cause a determinable changein the capacitance of the pressure capsule. Techniques for measuringthis capacitance or change in capacitance are known in the art. One suchscheme is shown by W. R. Polye in U.S. Pat. No. 3,858,097, which issuedDec. 31, 1974, while another is taught by C. Y. Lee in his commonlyassigned U.S. patent application Ser. No. 965,453, filed Nov. 30, 1978,both of which are expressly incorporated by reference.

The following discussions describe the method of manufacturing the abovedescribed pressure capsule 100 and consists primarily of four majorprocedures: (1) surface preparation which ensures the required surfaceflatness and smoothness of the respective quartz plates or discs 102,104; (2) silk screen printing and curing of the electrodes, contacts,ground shield, (3) silk screening of the frit glass and preglazing todrive off the organic binders contained in the frit glass material and(4) the vacuum sealing of the pressure capsule.

The fabrication process begins by preparing quartz blanks which in thepreferred embodiment are disc-like structures having a one inch diameterwith flat areas (see FIGS. 7 and 8) ground on opposing sides or ends.The parallel surfaces (120, 122, 124, 126) of the discs are also groundto ensure a determinable flatness. It has been found that a deviationfrom flat across these surfaces of the disc should be less than 5,000angstroms. After grinding, the quartz discs are cleaned and air fired to900° C. After air firing the electrical elements (electrodes, contactsand ground shield) of each disc are silk screened thereon and theelectrical elements cured. The material used for the electrodes andground shield is a metal organic ink A-1830 manufactured by theIngelhart Corporation, New York City, N.Y. The metal organic ink is aplatinum gold combination held in suspension by organic binders. Theprimary constituents of this metal organic ink comprises: 15.0% Au, 2.0%Pt and 0.066% Rh.

The first silk screen printing operation can place the ground shield 116on surface 126 of the quartz disc 104. This ground shield 116 is airdried in a suitable furnace (not shown). The air drying could beperformed between the temperature range of 100° C. to 150° C. and it hasbeen found that a drying time of 15 minutes is adequate. The furnace canbe a simple box type furnace of the type having a predetermined butvariable temperature therein and means for enriching the environment byinjecting oxygen (O₂) therein. Alternatively, the furnace may be anautomated belt furnace having a plurality of temperature zones andhaving the above oxygen enrichment features and further having conveyormeans or a belt for transporting the parts to be processed into and outfrom these temperature zones.

After the air drying step, the ground shield 116 may first be cured oralternatively returned to the silk screen printer prior to curing wherethe C_(p) -C_(r) electrodes, i.e. electrodes 112 and 114, are printed onthe flat surface 124 and then this combination is now similarly airdried. The disc 104 is returned to the furnace for the electrode curingprocedure. The curing procedure is a two-step process which firstrequires that the organic binders in the ink forming the electrodes andother electrical elements be driven off, and secondly to securely attach(i.e., fused) the remaining material to the quartz. During the curingprocess, the thickness of the remaining electrode materials are reducedto a thin film having an average thickness of 18KA. The curing processcan be implemented as a number of distinct procedures as illustrated inFIG. 12 or as an equivalent longer single procedure as illustrated inFIG. 13. The choice of implementation will depend primarily upon thetype of furnace employed.

Reference is again made to the curing procedure illustrated in FIG. 12.This procedure is designed to be implemented in a box type of furnaceand shows the desired electrode temperatures as a function of time. Theprocedure comprises a first curing step which is a low temperaturefiring (solid lines) followed by a high temperature firing (dashedlines). The quartz plate having the designated electrodes thereon isinserted into the furnace which is preheated to a first temperature suchas 300° C. The quartz plate (and electrode) rapidly achieves this firsttemperature (Point A).

The furnace temperature is increased to a higher level. The electrodetemperature gradually approaches this higher temperature (Point B) andduring this time, a large percentage of the hydrocarbons in theelectrode material are burned off. It is noted in passing that thefurnace should be adequately ventilated to permit these fumes to escape.The furnace temperature is again elevated to an even higher temperaturesuch as 600° C. The electrode temperature will again increaseapproaching 600° C. (Point C). During this phase, additional organicmaterial is liberated from the electrodes (or other electrical elementssuch as the ground shield 116 being processed). Upon achieving thishigher temperature, the electrode is slowly cooled as illustrated inFIG. 12. As will be shown later, it is possible to eliminate the coolingand proceed directly to the high temperature firing.

After cooling, the quartz plate is placed into a furnace which ispreheated to about 900° C. (more specifically 884° C.). This temperaturecorresponds to the electrode fusing temperature. Depending upon theamount of air within the furnace it may be necessary to enrich theatmosphere therein with oxygen. The high temperature firing process ofFIG. 12 was performed in a 1-5 torr oxygen enriched environment. Afterthe electrode (i.e., quartz plate) achieves this temperature (Point D)the quartz plate is cooled by reducing the furnace temperature asindicated in FIG. 12.

Reference is now made to FIG. 13 which illustrates an alternate curingprocedure designed for a multi-zone belt furnace. The furnace (notshown) comprises at least five temperature zones and a moveable belt totransport the quartz plates being processed into the various zones. Thetemperature of the various zones and belt speed are chosen to achievethe results of the previously described segmented curing process, thatis, to permit sufficient time for the electrodes to heat up and achievea temperature which encourages the burning off of the organic materials(approximately 600° C.) and then to permit the electrode material tofuse to the quartz by controlling the rate of temperature increase toapproximately 900° C. It should be appreciated that the burn-offtemperature, the fusing temperature and heating time in the proceduresillustrated in FIGS. 12 and 13 are interrelated so that the aboveprocedures are not limited to the exact time-temperature profiles areshown. As an example, it is conceivable that the driving off of theorganic binders can be achieved between 500° C. and 700° C. withappropriate changes in the heating time. With similar changes in heatingtimes the fusing temperature might vary between 800° C. to 1,000° C. Inaddition, the temperature and heating times could be affected by thevarying thermal mass presented by different quantities of quartz discsbeing processed, holding fixtures (if required) and the size of thefurnace.

The curing process of FIG. 13 has been implemented by setting the beltspeed to speeds between 3 and 5 inches per minute and by preheating thezones to the following temperatures: zones 1 and 2: 350°-625° C., zone3: 650°-800° C., zone 4: 950°-1000° C., and zone 5: 925°-950° C. Thefollowing zonal temperatures and belt speed appears to yield goodquality cured electrodes; zones 1 and 2: 350° C., zone 3: 650° C., zone4: 950° C., zone 5: 925° C., and belt speed: 3.5 inches per minute. Thefurnace employed to achieve the temperature profile shown in FIG. 13 isa continuous belt furnace having 5 temperature zones with each zoneapproximately 30 inches long. This furnace is manufactured by BruceIndustrial Controls, Inc., North Billerica, Mass. and is sufficientlylarge that oxygen enrichment is not a necessary requirement. In additionto the five heating zones within the curing furnace, the furnacecontains a water-cooled heat exchanger located after zone 5. The quartzplates are cooled by moving them through this heat exchanger portion.

A further step towards achieving a fully automated mass productioncuring procedure is to eliminate the handling of the quartz plates afterthe printing step or at least between the air drying step and theintroduction of the quartz plates into the curing furnace. This can beachieved by having the air drying furnace located at the input end butspaced from the multi-zone curing furnace. The conveyor belt is thenextended through the air drying furnace. In this manner, after theelectrode material has been printed on the quartz plate, the plate isplaced on the conveyor and moved through the drying furnace for therequired drying time. These plates will be cooled somewhat as they aremoved through the space between both furnaces. After this, the platesproceed into the curing furnace and are cured as described above.

The disc 104 (and/or second disc 102) is returned to the silk screenprinter and the ring of frit glass 106 is printed around the electrodes112, 114 having a diameter slightly larger than the outside diameter ofthe circular portion of the ground electrode 110. The preferredembodiment uses a slurry frit (P-1015) manufactured by the VittaCorporation of Danbury Ct. This frit material was chosen because it hasa coefficient of thermal expansion which is compatible to thecoefficient of thermal expansion of fused quartz. The frit glassmaterial has a viscosity of approximately 200 centi-stokes which permitsthe silk screening of the frit around the combination C_(p) -C_(r)electrode. The printed frit is then air dried at 120° C. for about 15minutes. The thickness of the frit is controlled using standard silkscreening techniques, such as controlling the size of the mesh used andthe amount of frit glass material printed. Following the drying of thequartz disc having the ring of frit glass 106 imprinted thereon, thequartz disc 104 is placed face-up onto a carrier and again loaded intothe furnace having an oxygen enriched atmosphere. The temperature of thefurnace is increased from room temperature to about 900° C., moreparticularly 884° C. The temperature of the furnace is controlled sothat the plate 104 temperature increases from ambient to 900° C. andtakes between 6 to 7 minutes. Alternatively, the high temperature curingprocedure (FIG. 12) can be used here. This procedure drives off theorganic binders in the frit material and reduces the frit to a glassphase. The significance of the preglazing procedure can be appreciatedin the following context. If the quartz plates are positioned face toface and sealed without a preglazing procedure, at some time during thesealing process, the organic binders, which are still in the fritmaterial, will produce a bubble structure. The bubble structure will benon-uniform and vary from one pressure capsule to the next, thus makingit virtually impossible to achieve a repeatable and mass producedpressure capsule 100. However, the preglazing of the frit glass materialprior to sealing of the capsule 100, maximizes the amount of organicbinders liberated from the frit glass and reduces the bubble structurecaused by the reduction of the metal oxides within the frit, thusenhancing the capsule to capsule uniformity. By reducing the bubblestructure to a minimum, one inherently achieves a means of temperaturestabilizing the pressure capsule 100. A substitute preglazing procedurewould be to subject the frit to the time-temperature profile or modifiedprofile such as illustrated in FIG. 13.

The second quartz disc 102 having the circular ground electrode 110 andcontact pads 128 are similarly printed, air dried and air fired andcured.

The vacuum sealing of the pressure capsule 100 is accomplished in thefollowing manner. The quartz disc 104 which contains the C_(p) -C_(r)electrode and the quartz disc 102 containing the ground electrode 110are placed within a registration fixture 175. The registration fixture175 is illustrated in FIGS. 9-11 and is discussed in detail later. Theregistration fixture 175 permits the two quartz discs 102 and 104 to beplaced face to face but rotated a determinable amount (90° to thepreferred embodiment) relative to one another. The registration fixture175 surrounds the pressure capsule 100 with a heat absorbing mass duringthe sealing process to produce a uniform radiation pattern for theheating and sealing of the pressure capsule 100. A single registrationfixture 175 or a plurality of such fixtures and associated pressurecapsules 100 are stacked on a quartz boat or moveable tray (not shown)having a thermocouple and loaded into the diffusion furnace (not shown).

Reference is now made to FIG. 14 which shows the temperature-pressureprofile used to vacuum seal each pressure capsule 100 and achieve adeterminable vacuum pressure therein. FIG. 14 illustrates the designedpressure profile and two temperature profiles both of which have yieldedacceptable results. The first profile (lines A and B of FIG. 14)requires a monotonic temperature increase over a predetermined periodfollowed by a cooling period. The second profile (lines C and B)illustrate a discontinuous temperature profile followed by the samecooling period.

After the registration fixture 175 containing an appropriate number ofunsealed pressure capsules 100 is loaded into a first ambienttemperature zone of a diffusion furnace (not shown) the furnace is thensealed and evacuated from atmospheric pressure to a first determinablepressure level. The magnitude of this first pressure level issufficiently low to encourage the outgassing of the organic materialswhich remain in the electrode or frit material. By removing these gasesand the bubbles which might form the temperature coefficient of thecapsule is stabilized. The first pressure level should be as low aspossible, however, 0.05 torr appears to be adequate. After the furnacehas stabilized at the first pressure level, the pressure within theunsealed pressure capsule 100, i.e. within the interior chamber 148(intermediate the quartz discs 102 and 104 and the circular frit disc106) will similarly arrive at this lower first pressure level. Theregistration fixtures 178 and capsules 100 are then subjected to one ofthe temperature profiles shown in FIG. 14. The following discussionillustrates the discontinuous temperature process and further presumesthat the sealing process is done within a diffusion furnace of the typehaving at least three temperature zones. The temperatures within eachzone will depend upon the characteristics of the metal-organic ink andthe frit glass material. The first temperature zone is maintained atambient temperature while the second temperature zone is maintained at alevel chosen to permit the outgassing of entrapped gases which remainwithin the frit material and metal-organic ink. The third temperaturezone is chosen at a level which will change the frit material to aviscous glass state in a predetermined time interval; in the preferredembodiment the third temperature approximately 900° C. At thistemperature, it was found that the Vitta frit material rapidly achievesa molten state.

The registration fixtures are then immediately moved (see FIG. 14) intothe second temperature zone wherein the temperature of the unsealedquartz capsules 100 will increase to the second temperature level. Theunsealed pressure capsules will be maintained within the secondtemperature zone for a period of time T₁ to allow entrapped gases todegas. It has been found that a degassing period of twelve to twentyminutes (depending on thermal mass or load) provides an adequatedegassing interval. After the degassing step, the registration fixtures(and unsealed pressure capsules 100) are now moved into the thirdtemperature zone for a period of time T₃, wherein the furnace pressureis increased to a slightly greater pressure such as 1-8 torr of O₂. Thisincreased pressure level decreases the possibility of causing the oxideswithin the frit material to reduce and become conductive. In addition,this pressure level determines the final pressure, (measured at roomtemperature) to be achieved within the capsule 100 after the capsule issealed and cooled. The rate of temperature increase of the capsules 100is monitored and the third temperature level should be attained within a6-7 minute interval. After the frit has achieved a molten state (PointD, FIG. 14) the pressure within the diffusion furnace is increased to athird pressure level approaching atmospheric pressure. It is notnecessary however, to achieve atmospheric pressure; a third pressurelevel of 700 torr is sufficient. At this pressure level, the increasedpressure provides a clamping force of approximately 4.5 Kg (10 lbs.) oneach flat surface of the pressure capsule 100 therein compressing thefrit 106 to its final height. Higher clamping forces however, can beachieved by increasing the pressure in excess of atmospheric. Byutilizing the pressure profile as shown in FIG. 14 to provide a clampingforce on the pressure capsule, negates the requirement for having largethermal masses such as large hold-down weights within the furnace,therein reducing the process time and permitting the rapid and massproduction of the pressure capsules. It is desirable however, tointroduce a small hold-down weight of approximately 270 gm. on top ofthe uppermost registration fixture to further secure the registrationfixture to the pressure capsules 100 during the entire vacuum sealingprocess and to initially compress the frit glass 106. After the pressurewithin the furnace is increased to the third pressure level, it israpidly decreased to an intermediate fourth pressure level such as 40torr. It is required that the fourth pressure level be greater than thepressure internal to the capsule 100. The registration fixtures 175 arethen moved to the first temperature zone and allowed to cool. It issufficient to let the pressure capsules cool to a temperature which isless than 470° C. At this temperature, the pressure within the furnaceis raised to atmospheric pressure and the sealed pressure capsulesremoved.

It should be noted that when the furnace pressure is raised to the thirdpressure level the pressure exterior to the pressure capsule issignificantly greater than the internal pressure of the interior chamber148. During this time, the viscous frit material will prevent anysubstantial oxygen penetration. As previously recited, after clamping,that is after the application, of the clamping pressure the furnacepressure was immediately reduced to a lesser fourth pressure leveltherein further minimizing the possibility of oxygen penetration withinthe interior of the pressure capsule. It should be apparent that at thethird pressure level, the clamping force will deform the capsule 100from its desired parallel configuration. The capsule deformation maycause portions of the molten frit glass 106 to achieve an uneventhickness since the frit glass 106 is by now attached to and moveablewith the quartz plates 102 and 104. If capsule 100 were permitted tocool in this deformed state, the measurement characteristics of thecapsule 100 would be effected. Consequently, the fourth pressure levelis chosen to permit the plates (102 and 104) to return to a parallelorientation.

Reference is again made to FIGS. 9, 10 and 11 which illustrate thefeatures of the registration fixture 175 and illustrate a method ofstacking a plurality of registration fixtures one on to another topermit the mass vacuum sealing of a plurality of pressure capsules 100.Registration fixture 175 comprises a plurality of thin metal plates 176fabricated from a steel such as the rolled alloy 330 (RA 330) having anupper surface 180 and a parallel lower surface 182. Each surfacecontains a pressure capsule receiving cavity 184 or 186, respectively.The dimensions of the pressure receiving cavities 184 and 186 conform tothe shape of the quartz plates or discs 102 and 104 and are sized toclosely receive each disc 102 and 104. In addition, the depth "d" ofeach pressure capsule receiving cavity (184, 186) is chosen so that whenthe quartz disc having electrodes or frit imprinted thereon is placedtherein, a portion of the quartz discs will protrude therefrom. Thepressure capsule receiving cavities 184 and 186 on the upper and lowersurface 180 and 182 of each plate 176 are oriented relative to oneanother to permit the alignment of the flat ends 132, 150 (i.e.,cut-outs) to the electrical contacts 120, 140 and 146. In the preferredembodiment, the pressure capsule receiving cavities 180 and 182 arerotated 90° relative to each other to permit the desired perpendicularplacement of the plurality of electrical contacts.

The loading or the stacking of each pressure capsule 100 within theregistration fixture is performed in the following manner. A cured andpreglazed disc such as disc 104 is placed into the upper pressurecapsule receiving cavity 182 of one of the registration fixture platessuch as plate 176a. The second cured disc 102 is placed within the lowerreceiving pressure capsule receiving cavity 186 of a second plate suchas plate 176b. The plates 176a and 176b having the quartz plates 102,104 respectively with the electrode surfaces protruding therefrom andare laid one onto the other to achieve the 90° mounting relationship ofthe electrodes 110, 112, and 114. By repeating the above stackingprocedure a plurality of pressure capsules 100 can be assembled instacked relationship within the registration fixture 175.

To achieve the alignment of each metal plate 176 and therefore thealignment of each disc 102 to the opposing disc 104 every metal plate176 is fabricated with an set of alignment holes 190 adapted to receivean alignment pin 192. The alignment pin 192 is fabricated from the samemetal used for the metal plate 176. To enhance the uniform heating ofthe pressure capsule 100 by the registration fixture 175, it ispreferable to heat treat the registration fixture 175 (i.e., each plate176) to develop an oxide layer throughout.

Reference is now made to FIG. 15 and 16 which illustrates an alternateembodiment of the pressure capsule 100. In particular, there is shown acircular pressure capsule 154 having a circular upper plate or disc 156and circular lower plate or disc 158. The arrangement of the electrodes110, 112, and 114 thereon remains as previously described in thepreferred embodiment. However, the electrical contacts 140 and 146 areno longer oppositely situated but rather skew oriented relative to oneanother; the circular ground electrode 110 is in electricalcommunication with only one electrical contact 128 which is uniformlypositioned between contacts 140 and 146. In addition, the upper plate ordisc 156 contains a single cut-out 160a while the lower plate or disc158 contains a plurality of cut-outs such as cut-outs 160b, and c. Eachcut-out 160a-c is located on its respective disc to expose an oppositelysituated electrical contact (128, 140 or 146) located and the opposingdisc.

Many changes and modifications in the above described embodiment of theinvention can of course be carried out without parting from the scopethereof. Accordingly, that scope is intended to be limited only by thescope of the appended claims.

Having thus described the invention, which is claimed is:
 1. A method offabricating a pressure capsule comprising:1.1 providing two flexiblequartz plates having electrodes thereon, wherein the electrodes form theelements of a pressure sensitive capacitor;
 1. 2 applying a dielectricsealing glass material, having a predetermined viscosity, to at leastone of said quartz plates in the shape of a peripheral seal;1.3positioning said quartz plates with said dielectric therebetween in asubstantial parallel orientation, at an initial spacing determined bythe height of said dielectric; 1.4 decreasing the local pressureproximate the capsule from atmospheric pressure to a lower firstpressure level thereby similarly reducing the pressure within thecapsule to the first pressure level; 1.5 monotonically increasing thelocal temperature to a temperature level to reduce the dielectric to amolten glaseous state to encourage outgassing of an organic mattercontained in the electrode or dielectric to a molten state; 1.6increasing the local pressure, in an oxygen enriched environment, to asecond pressure level between the range of 1-8 torr of oxygen, toprevent any oxide material in the dielectric from becoming conductivethereby causing an electrical short circuit between the electrodes; 1.7varying the separation between the plates by clamping the platestogether reducing the thickness of the dielectric by increasing thelocal pressure to a third pressure level after the dielectric hasattained said molten state.
 2. The method as recited in claim 1 whereinsaid third pressure level is approximately atmospheric.
 3. The method asrecited in claim 1 further including the following steps:3.1 decreasingsaid local pressure to a fourth pressure level intermediate said firstand said third pressure levels; 3.2 cooling the capsule; 3.3 increasingsaid local pressure to atmospheric after the capsule has cooled.
 4. Themethod as recited in claim 1 wherein said second temperature level isbetween 650° C.-800° C. and said third temperature level is between 800°C.-1,000° C. and said first pressure level is between 8.0 torr andabsolute vacuum, and said second pressure level is between 1.0 torr and40 torr; and said third pressure level is between 500 torr andatmospheric pressure and said fourth pressure level is between 500 torrand 1.0 torr.
 5. The method as recited in claim 1 wherein;said secondtemperature level is 700° C. and said third temperature level is 900° C.and said first pressure level is 0.5 torr and said second pressure levelis 1.0 torr and said third pressure level is 700 torr and said fourthpressure level is 40 torr.
 6. The method as recited in claim 5 includingthe additional step of compressively loading said plates prior to thestep of decreasing the local pressure.
 7. The method as defined in claim1 wherein said predetermined viscosity is approximately 200centi-stokes.
 8. A method of mass producing a plurality pressurecapsules comprising:8.1 providing a pair of flexible quartz plates foreach capsule having electrodes thereon, wherein the electrodes for theelements of a pressure sensitive capacitor; 8.2 applying a dielectricsealing glass material having a predetermined viscosity to at least oneof said quartz plates of each capsule in the shape of a peripheral seal;8.3 positioning said quartz plates of each capsule with said dielectrictherebetween in a substantial parallel orientation at an initial spacingdetermined by the height of said dielectric; 8.4 placing the capsule inthe first temperature zone having a first temperature level which issubstantially ambient; 8.5 increasing the capsule temperature by movingthe capsule to a temperature zone having a higher temperature level topermit the dielectric to reduce the dielectric to a molten glaseousstate;
 8. 6 decreasing the furnace pressure from atmospheric pressure toa lower first pressure level thereby similarly reducing the pressurewithin each capsule to the first pressure level;8.7 permitting entrappedgases within the dielectric to outgas; 8.8 clamping the plates byincreasing the furnace pressure to a second level to reduce thethickness of the dielectric after the dielectric has attained saidmolten state; 8.9 decreasing said furnace pressure to a third pressurelevel intermediate said first and said second pressure levels; 8.10cooling the capsule by moving the capsule to said first temperaturezone; 8.11 increasing the furnace pressure to atmospheric pressure. 9.The method as recited in claim 1 or 8 wherein said step of clampingincludes increasing said furnace pressure to a third pressure level thatis less than or equal to atmospheric pressure.
 10. The method as recitedin claim 10 wherein said third pressure level is 700 torr.
 11. Themethod as defined in claim 8 wherein said predetermined viscosity isapproximately 200 centi-stokes.
 12. A method of fabricating a pressurecapsule comprising:providing two flexible quartz plates havingelectrodes thereon, wherein the electrodes form the elements of apressure sensitive capacitor; applying a dielectric sealing glassmaterial having a predetermined viscosity to at least one of said quartzplates in the shape of a perimetal seal; positioning said quartz plateswith said dielectric therebetween in a substantial parallel orientation,at an initial spacing determined by the initial height of saiddielectric; decreasing the pressure within the volume between saidplates and said dielectric to a first pressure level that issufficiently low to encourage outgassing of air entrapped within saiddielectric and the electrode material; heating said plates to reducesaid dielectric to a molten state; achieving the final spacing betweensaid plates by clamping said plates together to compress the thicknessof said dielectric by momentarily increasing the pressure proximate saidplates to substantially atmospheric pressure after said dielectric hasachieved a molten state.
 13. The method as defined in claim 12 furtherincluding, after the step of heating;preventing said dielectric fromexhibiting electrically conductive characteristics by increasing thepressure between said plates, in an oxygen enriched environment, to asecond pressure level of 1-8 torr of oxygen.
 14. The method as definedin claim 13 wherein said second pressure level is slightly greater thansaid first pressure level.
 15. The method as defined in claim 12 furtherincluding:permitting said plates to achieve a final parallel orientationand establishing a reference pressure between said plates bypressurizing the volume between said plates to an intermediate pressurelevel, wherein said intermediate pressure level is between 8 torr andatmospheric.
 16. The method as recited in claims 12 or 15 wherein saidintermediate pressure level is 40 torr.
 17. The method as defined inclaim 12 wherein said predetermined viscosity is approximately 200centi-stokes.